One-humped Camel

The dromedary (''Camelus dromedarius'' or ;), also known as the dromedary camel, Arabian camel, or one-humped camel, is a large even-toed ungulate, of the genus '' Camelus'', with one hump on its back.

It is the tallest of the three species of camel; adult males stand at the shoulder, while females are tall. Males typically weigh between , and females weigh between .

The species' distinctive features include its long, curved neck, narrow chest, a single hump (compared with two on the Bactrian camel and wild Bactrian camel), and long hairs on the throat, shoulders and hump. The coat is generally a shade of brown. The hump, tall or more, is made of fat bound together by fibrous tissue.

Dromedaries are mainly active during daylight hours. They form herds of about 20 individuals, which are led by a dominant male. They feed on foliage and desert vegetation; several adaptations, such as the ability to tolerate losing more than 30% of its total water content, allow it to thrive in its desert habitat. Mating occurs annually and peaks in the rainy season; females bear a single calf after a gestation of 15 months.

The dromedary has not occurred naturally in the wild for nearly 2,000 years. It was probably first domesticated in the Arabian Peninsula about 4,000 years ago, or in Somalia where there are paintings in Laas Geel that figure it from 5,000 to 9,000 years ago. In the wild, the dromedary inhabited arid regions, including the Sahara Desert. The domesticated dromedary is generally found in the semi-arid to arid regions of the Old World, mainly in Africa and the Arabian Peninsula, and a significant feral population occurs in Australia. Products of the dromedary, including its meat and milk, support several North African tribes; it is also commonly used for riding and as a pack animal.

Etymology

The common name "dromedary" comes from the Old French ''dromedaire'' or the Late Latin ''dromedarius''. These originated from the Greek word ''dromas'', δρομάς (ο, η) ( GEN (γενική) ''dromados'', δρομάδος), meaning "running" or "runner",. used in Greek in the combination (''dromas kamelos''), literally "running camel", to refer to the dromedary. The first recorded use in English of the name "dromedary" occurred in the 14th century. The dromedary possibly originated in Arabia or Somalia and is therefore sometimes referred to as the Arabian or East African camel. The word "camel" generally refers either to the dromedary or the congeneric Bactrian; the word came into English via Old Norman, from the Latin word ''camēlus'', from Ancient Greek (''kámēlos''), ultimately from a Semitic source akin to Hebrew (''gamál'') and Arabic (''jamal'').

Taxonomy and classification

The dromedary shares the genus '' Camelus'' with the Bactrian camel (''C. bactrianus'') and the wild Bactrian camel (''C. ferus''). The dromedary belongs to the family Camelidae. The ancient Greek philosopher Aristotle (4th century BC) was the first to describe the species of ''Camelus''. He named two species in his '' History of Animals''; the one-humped Arabian camel and the two-humped Bactrian camel. The dromedary was given its current binomial name ''Camelus dromedarius'' by Swedish zoologist Carl Linnaeus in his 1758 publication ''Systema Naturae''. In 1927, British veterinarian Arnold Leese classified dromedaries by their basic habitats; the hill camels are small, muscular animals and efficient beasts of burden; the larger plains camels could be further divided into the desert type that can bear light burdens and are apt for riding, and the riverine type – slow animals that can bear heavy burdens; and those intermediate between these two types.

In 2007, Peng Cui of the Chinese Academy of Sciences and colleagues carried out a phylogenetic study of the evolutionary relationships between the two tribes of Camelidae; Camelini – consisting of the three ''Camelus'' species (the study considered the wild Bactrian camel as a subspecies of the Bactrian camel) – and Lamini, which consists of the alpaca (''Vicugna pacos''), the guanaco (''Lama guanicoe''), the llama (''L. glama'') and the vicuña (''V. vicugna''). The study showed the two tribes had diverged 25 million years ago (early Miocene), earlier than previously estimated from North American fossils.

The dromedary and the Bactrian camel often interbreed to produce fertile offspring. Where the ranges of the species overlap, such as in northern Punjab, Persia, and Afghanistan, the phenotypic differences between them tend to decrease as a result of extensive crossbreeding. The fertility of their hybrid has given rise to speculation that the dromedary and the Bactrian camel should be merged into a single species with two varieties. However, a 1994 analysis of the mitochondrial cytochrome b gene showed the species display 10.3% divergence in their sequences.

Genetics and hybrids

The dromedary has 74 diploid chromosomes, the same as other camelids. The autosomes consist of five pairs of small to medium-sized metacentrics and submetacentrics. The X chromosome is the largest in the metacentric and submetacentric group. There are 31 pairs of acrocentrics. The dromedary's karyotype is similar to that of the Bactrian camel.

Camel hybridization began in the first millennium BC. For about a thousand years, Bactrian camels and dromedaries have been successfully bred in regions where they are sympatric to form hybrids with either a long, slightly lopsided hump or two humps – one small and one large. These hybrids are larger and stronger than their parents – they can bear greater loads. A cross between a first generation female hybrid and a male Bactrian camel can also produce a hybrid. Hybrids from other combinations tend to be bad-tempered or runts.

Evolution

The extinct ''Protylopus'', which occurred in North America during the upper Eocene, is the oldest and the smallest-known camel. During the transition from Pliocene to Pleistocene, several mammals faced extinction. This period marked the successful radiation of the ''Camelus'' species, which migrated over the Bering Strait and dispersed widely into Asia, eastern Europe and Africa. By the Pleistocene, ancestors of the dromedary occurred in the Middle East and northern Africa.

The modern dromedary probably evolved in the hotter, arid regions of western Asia from the Bactrian camel, which in turn was closely related to the earliest Old World camels. This hypothesis is supported by the fact that the dromedary foetus has two humps, while in the adult male an anterior vestigial hump is present. A jawbone of a dromedary that dated from 8,200 BC was found in Saudi Arabia on the southern coast of the Red Sea.

In 1975, Richard Bulliet of Columbia University wrote that the dromedary exists in large numbers in areas from which the Bactrian camel has disappeared; the converse is also true to a great extent. He said this substitution could have taken place because of the heavy dependence on the milk, meat and wool of the dromedary by Syrian and Arabian nomads, while the Asiatic people domesticated the Bactrian camel but did not have to depend upon its products.

Characteristics

The dromedary is the tallest of the three camel species. Adult males range in height between at the shoulder; Guinness World Records
Largest species of camel
/ref> females range between . Males typically weigh between ; females range between . The distinctive features are its long, curved neck, narrow chest and single hump (the Bactrian camel has two), thick, double-layered eyelashes and bushy eyebrows. They have sharp vision and a good sense of smell. The male has a soft palate ( in Arabic) nearly long, which he inflates to produce a deep pink sac. The palate, which is often mistaken for the tongue, dangles from one side of the mouth and is used to attract females during the mating season.

The coat is generally brown but can range from black to nearly white. Leese reported piebald dromedaries in Kordofan and Darfur in Sudan. Piebald coloration in some camels is thought to be caused by the ''KIT^W1'' allele of the KIT gene, though there is likely at least one other mutation that also causes white spotting. The hair is long and concentrated on the throat, shoulders and the hump. The large eyes are protected by prominent supraorbital ridges; the ears are small and rounded. The hump is at least high. The dromedary has long, powerful legs with two toes on each foot. The feet resemble flat, leathery pads. Like the giraffe, dromedaries moves both legs on one side of the body at the same time.

Compared with the Bactrian camel, the dromedary has a lighter build, longer limbs, shorter hairs, a harder palate and an insignificant or absent ethmoidal fissure. Unlike the camelids of the genus ''Lama'', the dromedary has a hump, and in comparison has a longer tail, smaller ears, squarer feet, and a greater height at the shoulder. The dromedary has four teats instead of the two in the ''Lama'' species.

Anatomy

The cranium of the dromedary consists of a postorbital bar, a tympanic bulla filled with spongiosa, a well-defined sagittal crest, a long facial part and an indented nasal bone. Typically, there are eight sternal and four non-sternal pairs of ribs. The spinal cord is nearly long; it terminates in the second and third sacral vertebra. The fibula is reduced to a malleolar bone. The dromedary is a digitigrade animal; it walks on its toes, which are known as digits. It lacks the second and fifth digits. The front feet are wide and long; they are larger than the hind feet, which measure wide and long.

The dromedary has 22 milk teeth, which are eventually replaced by 34 permanent teeth. The dental formula for permanent dentition is , and for milk dentition. In the juvenile, the lower first molars develop by 12 to 15 months and the permanent lower incisors appear at 4.5 to 6.5 years of age. All teeth are in use by 8 years. The lenses of the eyes contain crystallin, which constitutes 8 to 13% of the protein present there.

The skin is black; the epidermis is thick and the dermis is thick. The hump is composed of fat bound together by fibrous tissue. There are no glands on the face; males have glands that appear to be modified apocrine sweat glands that secrete pungent, coffee-coloured fluid during the rut, located on either side of the neck midline. The glands generally grow heavier during the rut, and range from . Each cover hair is associated with an arrector pilli muscle, a hair follicle, a ring of sebaceous glands and a sweat gland. Females have cone-shaped, four-chambered mammary glands that are long with a base diameter of . These glands can produce milk with up to 90% water content even if the mother is at risk of dehydration.

The heart weighs around ; it has two ventricles with the tip curving to the left. The pulse rate is 50 beats per minute. The dromedary is the only mammal with oval red blood corpuscles, which facilitates blood flow during dehydration. The pH of the blood varies from 7.1 to 7.6 (slightly alkaline). The individual's state of hydration and sex and the time of year can influence blood values. The lungs lack lobes. A dehydrated camel has a lower breathing rate. Each kidney has a capacity of , and can produce urine with high chloride concentrations. Like the horse, the dromedary has no gall bladder. The grayish violet, crescent-like spleen weighs less than . The triangular, four-chambered liver weighs ; its dimensions are .

Reproductive system

The ovaries are reddish, circular and flattened. They are enclosed in a conical bursa and have the dimensions during anestrus. The oviducts are long. The uterus is bicornuate. The vagina is long and has well-developed Bartholin's glands. The vulva is deep and has a small clitoris. The placenta is diffuse and epitheliochorial, with a crescent-like chorion.

The penis is covered by a triangular penile sheath that opens backwards; it is about long. The scrotum is located high in the perineum with the testicles in separate sacs. Testicles are long, deep and wide. The right testicle is often smaller than the left. The typical mass of either testicle is less than ; during the rut the mass increases from . The Cowper's gland is white, almond-shaped and lacks seminal vesicles; the prostate gland is dark yellow, disc-shaped and divided into two lobes.
The camel epididymis interstitium revealed several blood vessels harboring special regulatory devices such as the spiral arteries, spiral veins, and throttle arterioles.

Health and diseases

The dromedary generally suffers from fewer diseases than other domestic livestock such as goats and cattle. Temperature fluctuations occur throughout the day in a healthy dromedary – the temperature falls to its minimum at dawn, rises until sunset and falls during the night. Nervous camels may vomit if they are carelessly handled; this does not always indicate a disorder. Rutting males may develop nausea.

The dromedary is prone to trypanosomiasis, a disease caused by a parasite transmitted by the tsetse fly. The main symptoms are recurring fever, anaemia and weakness; the disease is typically fatal for the camel. Brucellosis is another prominent malady. In an observational study, the seroprevalence of this disease was generally low (2 to 5%) in nomadic or moderately free dromedaries, but it was higher (8 to 15%) in denser populations. Brucellosis is caused by different biotype